The present invention is related to magnetic sensing of motion in a microfabricated device. More particularly the invention relates to a magnetic sensing unit for measuring displacements or flections on a nanometer scale.
The miniaturization of mechanical components and devices provides new applications in various fields and allows insight in and the use of a new world, the so called nano-world. At the end of the 20th century the basis for the age of micro- and nanomechanics has been created. Batch fabrication based on today""s chip manufacturing methods has been introduced which provides considerable potential for creating high-performance systems and devices at low cost. Applications in the field of mass data storage achieve much smaller storage devices and opens up the possibility of achieving storage densities in the range of hundreds of gigabits per square inch. In the field of microscopy, for example, the scanning tunneling microscope (STM) and the atomic force microscope (AFM) has been introduced successfully in recent years for atomic-scale surface analysis. Such microscopy techniques, in general called scanning probe microscopy (SPM), use a flexible cantilever of very small dimension whereby the cantilever is fabricated by micro machining techniques. The cantilever with a sharp tip is scanned across a surface of a sample and the displacement or the motion of the cantilever is detected in order to achieve an image having an atomic resolution. A variety of optical methods have been devised to detect the cantilever deflection. Typical forces between tip and sample range from 6 to 11 nN, and deflections as small as 0.001 nm can be detected. The three different ways of operation are contact mode, non-contact or dynamic force mode, and tapping mode which allow the detection of lateral, magnetic, electrostatic and Van der Waals forces. Also, such a cantilever can be used to write and read data.
Although the present invention is applicable in a variety of micro mechanical applications it will be described with the focus put on an application to cantilevers.
Today, several techniques are known to measure displacements or the motion of cantilevers used in scanning probe microscopy (SPM), for example, or other microfabricated devices.
The optical technique, also referred to as laser detection or optical beam deflection, uses either the reflection of a laser beam at the surface of a cantilever and therewith the change of the laser beam""s angle during deflection or the interference effects between the incident and reflected beams. The deflection of the cantilever is monitored by reflecting the laser beam off the cantilever into a photodiode. During scanning, an image can be formed by mapping this laser-detected deflection.
With a piezoresistive technique the change of resistance of a piezoresistive path defined at the surface of a flexed arm of the cantilever can be measured. In the article xe2x80x9cAtomic force microscopy using a piezoresitive cantileverxe2x80x9d, by M. Tortonese et al., Proc. of the Int""l Conf. on Solid State Sensors and Actuators, San Francisco, Jun. 24-27, 1991, pp. 448-451, the fabrication of a silicon cantilever beam with an integrated piezoresistor for sensing its deflection is described. A silicon on insulator material was used for the fabrication.
From U.S. Pat. No. 5,345,815 a microminiature cantilever structure is known having a cantilever arm with a piezoresistive resistor embedded close to the fixed end of the cantilever arm. Deflection of the free end of the cantilever arm produces stress in the base of the cantilever. That stress changes the piezoresistive resistor""s resistance at the base of the cantilever in proportion to the cantilever arm""s deflection. A resistance measuring apparatus is coupled to the piezoresistive resistor to measure its resistance and to generate a signal corresponding to the cantilever arm""s deflection.
U.S. Pat. No. 5,444,244 is related to a cantilever for a scanning probe microscope that includes a piezoresistor. A process of fabricating such a cantilever is further described, the process yielding a tip which has a high aspect ratio and a small radius of curvature at its apex. A combined atomic force/lateral force microscope including two or more piezoresistors responsive to both the bending and torsion of the cantilever is also disclosed.
However, piezoresistive cantilevers in spite of almost similar sensitivity as optical schemes, suffer from low frequency noise and temperature drift inherent to all semiconductor strain gauges. They require furthermore that the cantilevers be formed of single-crystal silicon.
IBM""s U.S. Pat. No. 5,856,617 describes an atomic force microscope (AFM) that uses a spin valve magnetoresistive strain gauge integrated on the AFM cantilever to detect its deflection. The spin valve strain gauge operates in the absence of an applied magnetic field. The spin valve strain gauge on the AFM cantilever is made of a plurality of films, one of which is a free ferromagnetic layer that has nonzero magnetostriction and whose magnetic moment is free to rotate in the presence of an applied magnetic field. In the presence of an applied stress to the free ferromagnetic layer due to deflection of the cantilever, an angular displacement of the magnetic moment of the free ferromagnetic layer occurs, which results in a change in the electrical resistance of the spin valve strain gauge. An electrical resistance detection circuitry coupled to the spin valve strain gauge is used to determine cantilever deflection.
Document WO 96 03641 is related to a scanning probe microscope assembly that has an atomic force measurement (AFM) mode, a scanning tunneling measurement (STM) mode, a near-field spectronomy mode, a near-filed optical mode, and a hardness testing mode for examining an object.
The European patent application EP 0 397 416 A1 describes an apparatus for the high resolution imaging of macromolecules and interactions involving macromolecules. The apparatus comprises a surface on which the macromolecule under test is placed and a plurality of fine probes. Means are provided for scanning each of the probes across a small area of the surface in such a way that the total output from the probes covers the whole surface. Means such as a scanning tunneling and/or atomic force detector are used to monitor the movement of the individual probes in a direction transverse to the surface and display means are used to display the transverse movement of the probes, being illustrative of the topography of the surface.
An other European patent application EP 0 306 178 A2 is related to an acceleration sensor including a cantilever beam having a free end to which a permanent magnet is attached. A pair of magnetic sensors, each consisting of a barber-pole type magnetoresistive sensing element, are arranged opposite to and symmetrically with respect to the magnet. The cantilever is bent and the magnet is moved according to an acceleration, which is detected as outputs from the magnetoresistive sensing elements.
The German publication DE 41 03 589 A1 is related to a sensor device with a mechanical resonant oscillation element. The structure is similar to that of the acceleration sensor mentioned in the preceding paragraph with the little difference that only one sensor element is arranged in the prolongation of the beam.
U.S. Pat. No. 4,954,904 is related to an apparatus and method for controlling the flying height of a head over a rotating medium, such as used in a rigid disk drive employing magnetic, magneto-optic or optical recording techniques. The flying height is controlled via magnetic attraction or repulsion to maintain a selected and substantially uniform flying height of the head with respect to the rotating medium.
The optical technique and the piezoresistive technique are the most widely used techniques today. But other techniques, like capacitive, piezoelectric, or thermal techniques, can also be used instead of the optical or piezoresistive technique to detect the deflection of a cantilever or microdevice. The capacitive or electrostatic technique measures the change in capacity of a capacitor formed by the cantilever and a fixed reference electrode. The thermal technique uses a current at different wiring levels, for example, to heat parts of a microdevice.
Deflection detection techniques which are external to the cantilever require time-consuming alignments. A variety of feedback mechanisms can be used to acquire data and to maintain proper tip position.
The combination and integration of microfabricated devices with electronically controlled functionalites facilitates nanotechnological applications, characterized by precise movements, increased sensing, and actuation.
It is an object of the present invention to overcome the disadvantages of the prior art.
It is another object of the present invention to provide an alternative measuring method with an increased sensitivity.
It is still another object of the present invention to provide an integrated system for measuring displacements on a microfabricated device without the need of additional equipment for alignment.
It is further object of the present invention to present a detection system which is simple implementable and can be fabricated at low-cost
These objects of the invention are achieved by the features of the enclosed claims. Various modifications and improvements are contained in the dependent claims.
The present invention provides a contactless magnetic sensing system for measuring relative displacements on a nanometer scale. Such a system can be used in a microdevice (i.e., a microfabricated device). The invention is based on the measurement of the field B of a magnetic dipole located on a moveable part by use of a sensitive magnetic sensor placed on an adjacent fixed part. The magnetic sensor can also be arranged at the moveable part and the magnetic dipole, also referred to as magnetic element, at the fixed part. However, the magnetic sensor should be positioned properly with respect to the magnetic element in order to take advantage of the largest field gradient dB/dz, of the field and thus optimize the sensitivity. The sensitivity of such a magnetic sensing or detection system according to the present invention can be at least 10 times better than for the known piezoresistive technique and offers the advantage over optical techniques to be fully integrated on a device without the need of special optical access. The magnetic sensing or detection system can easily be implemented into Si technology with standard photolithography. Different magnetic materials, geometries, and sensing configurations can be proposed to optimize further the sensitivity.
As a first embodiment it is proposed to measure the deflection of an investigation and/or manipulation device. For that, the investigation and/or manipulation device comprises a cantilever, also referred to as moveable part, a magnetic element generating a magnetic field, and a magnetic sensor detecting it. The moveable part is attached to the fixed part and further comprises a free end with a tip. The magnetic element is located on the moveable part and the magnetic sensor on the fixed part. On the other hand it is also possible that the magnetic sensor is located on the moveable part and the magnetic element on the fixed part. At least one magnetic element and at least one magnetic sensor form a magnetic sensing unit. The magnetic sensor and/or the magnetic element can be integrated into the investigation and/or manipulation device such that they form an integral part of the investigation and/or manipulation device. The magnetic element and the magnetic sensor are arranged relative to each other such that when the moveable part is displaced the change of the magnetic field at the magnetic sensor is detectable by use of the magnetic sensor. Subsequent measuring facilities process the information obtained by the sensor and derive a characteristic value that is representative of the displacement or torsion of the investigation and/or manipulation device.
However the field of applications extends to a broader range of devices where relative motion or positioning needs to be controlled, and possible in devices where moving parts need a fast and sensitive feedback, e.g. flying heads or actuators.
If the magnetic sensor can be arranged opposite to the magnetic element, then the advantage occurs that the field of the magnetic element penetrates the magnetic sensor and a field gradient, preferably the maximal field gradient, can be detected and determined.
The magnetic sensor can be used for contactless detection which is especially advantageous since no frictional losses occur and the sensitivity is optimized. The deflection or motion of the moveable part is free and not damped or hindered.
When a large portion of the moveable part is separated from the fixed part by a gap, then the advantage occurs that a wide and free deflection or motion of the moveable part can be guaranteed with an enlarged measurable range.
The magnetic element can be a permanent magnet, which has the advantage to provide a constant magnetic field. There is no need for an external magnetic field nor for a built-in solenoid with additional wires. Hence no current is required which could lead to unwanted heating effects.
The magnetic element comprises a magnetic layer that can be made of Fe, Fe2O3, Ba, Co, Cr, Mg, Mn, Ni, Pt, Sr, V, or alloys thereof, or made of one of the following components: AlNiCo, FeCoCr, FeCoV, FeCoVCr, FeNiCo, NdFeB, SmCo. Also, the magnetic layer can comprise crystalline or amorphous metals, e.g. an alloy of AlNiCo-type, a platinum-cobalt alloy, an iron-cobalt-vanadium (chromium) alloy, a chromium-iron-cobalt alloy, a rare earth cobalt alloy, or a rare earth iron alloy. The magnetic layer can be made of a plurality of materials and components, preferably made of a hard magnetic material as listed above, whereby these materials and components can be adapted according to the application.
If the magnetic sensor can be integrated into or on top of the flexible or fixed part, such that the magnetic sensor is an integral part of the microdevice, then the advantage occurs that the magnetic sensor is pre-installed and does not need to be aligned before use.
If the microdevice comprises silicon and the magnetic sensor is integrated into the silicon, then the advantage occurs that Si technology with standard photolithography can be applied.
The magnetic sensor can be a Hall sensor, a magnetotransistor, magnetodiode, or a giant magnetoresistive sensor. This shows the advantage that several types of sensor can be used.
The magnetic sensor can be arranged such that the magnetic sensor and a subsequent measuring device or system are able to determine a field gradient of the magnetic field. By this subsequent measuring device or system a characteristic value that is representative of the respective deflection or motion can be derived for further processing.
The moveable part and the fixed part can be made of the same material. This might be advantageous for a simple fabrication process. But on the other hand, the moveable part and the fixed part can also be made of different materials, whereby the flexible and fixed part have different mechanical or physical properties.
If the moveable part is attached to the fixed part by attachment means, preferably by a leg, then the advantage occurs that the moveable part shows a free and smooth displacement. If the attachment means comprises an aperture, such as a hole or a slit, or one or more constrictions, then the effective width of the attachment means can be reduced leading to a reduction of the spring constant for higher sensitivity. Similarly, the thickness of the moveable part can be adjusted for optimal sensitivity.
It is especially advantageous if the magnetic element and the magnetic sensor are arranged within the microdevice such that a mechanical amplification increases the sensitivity of the magnetic sensing system. This can be achieved if the moveable part projects into the fixed part of the microdevice. If the moveable part and the fixed part are arranged in the same plane and more length of the moveable part projects into the fixed part than protrudes out of the fixed part, then the sensitivity is increased advantageously because of the mechanical amplification ratio.
When the microdevice comprises a plurality of magnetic sensors and at least one magnetic element forming together a magnetic sensing unit and further the magnetic sensors are arranged in the vicinity of the magnetic element, then the advantage occurs that the signal amplitude by use of multiple sensors can be increased. On the other hand, when the microdevice comprises a plurality of magnetic elements and at least one magnetic sensor forming also together a magnetic sensing unit and further the magnetic elements are arranged in the vicinity of, or around the magnetic sensor, then the advantage occurs that the sensitivity can be increased without an increase of noise, as it would occur in other systems.
An arrangement of such magnetic sensing units for a magnetic detection system can be realized by a meander-shaped gap that separates the moveable part form the fixed part and therewith the magnetic element(s) from the magnetic sensor(s), respectively. The magnetic sensors and elements can be arranged appropriately in a row within the meander. It is also possible is to arrange at least two magnetic sensors in the vicinity of one magnetic element, which is useful for the detection of torsion. It should be noted that there are several possibilities to arrange a magnetic sensing unit. In the most cases the design depends on the application.
It is also possible that the microdevice comprises a plurality of moveable parts and at each moveable part at least one magnetic element and at least one magnetic sensor. A plurality of detection systems, e.g. for different measurements, can be arranged or combined within such a microdevice.